1. Field of the Invention
The present invention relates to pressure swing adsorption (PSA) processes. PSA is a commonly used process for the purification of gases. Exemplary applications include separation of hydrogen from gas mixtures, separation of helium from natural gas, purification of landfill gas, and air separation for production of oxygen, nitrogen and/or argon.
2. Discussion of the Background
Many PSA systems are limited by their very large product and raffinate gas flow fluctuations. These fluctuations require sizeable storage or surge tanks to dampen the flow fluctuation adequately to allow proper function of downstream process equipment connected to the PSA system.
Industrial-scale gas separations have traditionally been executed using PSA cycles possessing at least one pressure-equalizing step to enhance pressurized product fractional recovery at a given purity. In PSA cycles, increased fractional recovery decreases the amount of gas rejected to the raffinate surge tank, and ensures a more nearly continuous flow of pressurized product gas. Cycles having three or more equalizations are known. Another step taken to reduce flow pulsation in the art is to operate cycles having many equalizations and many vessels in a single process train. An example of a PSA system having many vessels and many equalization steps is U.S. Pat. No. 3,986,849 to Fuderer, et al., which describes process trains possessing as many as ten adsorbent vessels and fifty-five valves. In industrial applications, the high energy and operating costs associated with loss of recoverable product has usually outweighed the considerable increase in complexity associated with more complex PSA cycles having one or more pressure equalizations, except for very large plants. Thus, most plants employ extremely large surge tanks for both pressurized product and raffinate gas.
PSA systems of all types, but especially those having multiple equalizations, are also subject to severe limitations due to their very high complexity and attendant high parts count. Not only does this complexity significantly increase the probability of a component failure, it also significantly increases the system size, assembly time, and material cost. Most PSA systems are single point of failure systems, with notable exceptions being the processes revealed in U.S. Pat. No. 4,234,322 to De Meyer et al. and U.S. application Ser. No. 10/269,064. Even in the exemplary processes, the PSA plant must eventually be shut-down to conduct maintenance on the defective component. Such shutdowns are extremely undesirable as they incur a significant amount of lost production time for the entire process facility. Further, when the PSA is connected to a high temperature process such as a hydrocarbon steam reformer, autothermal reformer, partial oxidation reformer, ammonia synthesis plant or ethylene cracker, the lifetime of the connected process equipment may be greatly reduced due to the high mechanical stresses incurred during a shutdown and restart event.
U.S. Pat. No. 6,051,050 to Keefer et al. describes systems using multiple rotary PSA modules in parallel in order to achieve greater overall system capacity, but fails to disclose a method or strategy for operating these modules in the event of a malfunction. The rotary modules of the Keefer et al. patent are quite different than those accepted in industrial practice, and are not subject to the same type of single point valve failure as valved PSA apparatuses. Their mode of failure is through gradual seal failure. The modules of the Keefer et al. patent also have a very large number of active beds, and they are thus less concerned with variations in product and raffinate gas flowrate pulsation. The low-pulsation rotary modules of the Keefer et al. patent and the similar inventions described in U.S. Pat. No. 5,112,367, U.S. Pat. No. 5,268,021, and U.S. Pat. No. 5,366,541 suffer from inevitable leakage due to their use of sliding seals. This leakage results in reduced purity and product recovery, as well as maintenance problems due to limited seal lifetime. High pressure exacerbates these problems, making rotary modules less desirable for industrially-important separations than typical valved PSA apparatuses.
Because of the extremely large size of typical valved PSA systems and their very high cost it has remained extremely undesirable to provide backup PSA capacity to prevent process shutdowns, especially for valved PSA systems having pressure equalizations and large numbers of adsorbent beds, with their attendant high complexity.
The inventors hereby incorporate by reference in their entirety an improved apparatus for advanced PSA systems that greatly reduces the complexity of the PSA apparatus employing pressure equalizations set forth in U.S. application Ser. No. 10/269,067, and methods for executing PSA cycles that dramatically reduce the number of valves required to execute PSA cycles set forth in U.S. application Ser. No. 10/269,064.